1. Field of the Invention
The invention relates to a driving force transmitting apparatus including a friction clutch in which friction sliding between clutch plates is lubricated with a lubricant.
2. Description of the Related Art
A driving force transmitting apparatus has been known which is, for example, mounted in a four-wheel drive vehicle enabling switching between a four-wheel driving state and a two-wheel driving state and which includes a friction clutch in which frictional sliding between clutch plates is lubricated with a lubricant (see, for example, Japanese Patent Application Publication No. 2009-269605 (JP 2009-269605 A).
The driving force transmitting apparatus (final drive unit) described in JP 2009-269605 A includes an outer plate carrier and an inner plate carrier that can rotate on the same rotation axis relative to each other, a friction clutch arranged between the outer plate carrier and the inner plate carrier, and a housing cover that houses the outer plate carrier and the inner plate carrier.
The friction clutch is a wet multi-disc clutch having a plurality of outer plates that rotates along with the outer plate carrier and a plurality of inner plates that rotates along with the inner plate carrier. Frictional sliding between the plates is lubricated with a lubricant.
In the four-wheel drive vehicle described in JP 2009-269605 A, during the two-wheel driving, torque transmission to a propeller shaft (intermediate shaft) that transmits a driving force in a front-rear direction of the vehicle is blocked in a switching apparatus located on a front side of the vehicle and in a driving-force transmitting apparatus located on a rear side of the vehicle. Consequently, during the two-wheel driving, even while the vehicle is traveling, rotation of the propeller shaft is stopped, suppressing traveling loads resulting from rotational resistance of the propeller shaft. Thus, fuel economy performance is improved.
During the four-wheel driving when the driving force of an engine is transmitted to the rear side through the propeller shaft, a lubricant is fed to between the set of the outer plates and the set of the inner plates in the friction clutch. Thus, the lubricant pumped up from an oil pan as a result of rotation of the outer plate carrier passes through an oil receiving pocket and an oil guide hole formed in a housing cover, and reaches the inner plate carrier. The lubricant having reached the inner plate carrier is fed to between the set of the outer plates and the set of the inner plates due to a centrifugal force.
In the driving force transmitting apparatus described in JP 2009-269605 A, during the four-wheel driving, the lubricant is pumped up from the oil pan as a result of rotation of the outer plate carrier. Thus, the rotational resistance of the outer plate carrier increases particularly when the vehicle travels at high speed. If the driving force transmitting apparatus described in JP 2009-269605 A is applied to a four-wheel drive vehicle in which the rotation of the propeller shaft is not stopped during the two-wheel driving, more than a needed amount of lubricant is fed to between the set of the outer plates and the set of the inner plates. Then, a drag torque resulting from the viscosity of the lubricant increases the rotational resistance, degrading fuel economy.